The invention relates to an apparatus for and method of holding pins. More especially, but not exclusively, the invention relates to holding pins in a pin-head as widely used in the field of chemistry and biotechnology for microarraying and other applications.
Microarraying is a technique in widespread use. Conventional microarraying is based on standard multi-well plates having a 4.5 mm grid and 384 wells, although other sizes are available. Liquid samples are stored in the wells of a well plate. The liquid may be assays or any other biological or chemical sample of interest. Sub-samples of the liquid within the well plates are carried to and deposited on a spotting surface as required. Usually many such deposits are needed and microarraying is a process whereby multiple deposits can be made simultaneously and under machine automation.
FIG. 1A of the accompanying drawings shows schematically a bed of an exemplary microarraying apparatus 100. A number of well plates 110 are shown on the apparatus bed. The well plates 100 contain liquid to be spotted onto an array of microscope-type slides 120. The apparatus has a translatable head mechanism 130 which carries and positions the pin holder 140 in three orthogonal axes x, y and z.
FIG. 1B of the accompanying drawings shows schematically a more detailed view of a conventional pin holder 140 designed to carry a 6xc3x974 rectangular array of pins 170. Each of the pins 170 is guided by an upper hole 180 and a lower hole 190 within the pin holder 140 so as to remain nominally vertical. The separation of neighboring holes matches that of the well plate spacing. The pins 170 are free to slide vertically within the pin holder 140 and a collar 175 provides an abutment for the pins 170 to define the bottom point of the pins 170.
In operation, the translatable head mechanism 130 is initially positioned so as to align the pins 170 with the required section of well plate 110. The pin holder 140 is then driven by the head mechanism 130 so as to partially immerse the pins 170 in the liquid to be spotted. Surface tension ensures that samples of fluid remain on the pins 170 as they are lifted away from the well plate. The pin holder 140 is then carried by the head mechanism 130 to the required location for spotting where it is again driven downwards to deposit some or all of the carried fluid at the chosen location. This can be achieved by bringing the pins 170 into direct contact with the surface 120. To avoid the necessity of precise vertical positioning of the pin holder 140 the pins 170 are free to slide vertically so as to limit the force applied to the spotting surface 120 as the pins 170 contact it. The head mechanism 130 may be lifted and re-positioned for further spotting with the fluid remaining on the pins 170, or it may be lifted and returned to the well plates 110 for re-coating with different fluid samples before further spotting. Typically, an extended and pre-programmed series of spotting operations will be undertaken automatically by the microarraying apparatus.
FIG. 2A of the accompanying drawings shows the resulting fluid deposition pattern that would occur from a single spotting operation with the pin holder 140 shown in FIG. 1B. There are 24 spots in a regular rectangular array and with a pitch which matches the well plate spacing. It is conventional to deposit a higher density of spots on the spotting surface 120 by repositioning the head 130 to a position slightly displaced from the initial spotting position for further spotting.
FIG. 2B of the accompanying drawings shows the resulting fluid deposition pattern that would occur from two further spotting operations, each slightly displaced from the previous.
FIG. 2C of the accompanying drawings shows the resulting fluid deposition pattern that would arise from many closely spaced spottings. Each of the individual boxes schematically represents a dense grid of spottings generated by a single pin. In order to reliably reproduce these dense grids, which typically comprise an 11xc3x9711 square grid of spots within the 4.5 mm pin separation, the pins 170 must be fixed such that their tips maintain the standard 4.5 mm spacing to a certain degree of accuracy, typically about 30 xcexcm or better. The characteristic effect of a single misaligned pin in the spot pattern of FIG. 2C is shown as the uniformly displaced group of spots 230.
FIG. 3A and FIG. 3B of the accompanying drawings show, in grossly exaggerated form, two possible sources of pin misalignment error. FIG. 3A shows an example where the upper hole 180 and the lower hole 190 are not axially aligned. FIG. 3B shows an example where the upper hole 180 and the lower hole 190 are not co-parallel. These alignment errors require the guiding holes 180, 190 to be oversized to allow free movement of the pins. However, these oversized holes themselves can lead to spotting errors in cases where the holes 180, 190 are relatively well aligned such that a pin becomes free to move away from the vertical and rattle within the pin holder. This results in scatter about the otherwise regular spot pattern associated with the rastering of the affected pin.
The high-tolerance machining required to minimize the problems associated with misalignment of the holes in the pin holder leads to high manufacturing costs. Conventional drilling techniques are unable to provide the required accuracy and jig grinding is necessary. The cost of jig grinding each hole in a pin holder of the type described above is significant. With a pin holder containing 48 holes the machining cost of the pin holder makes up a significant proportion of the overall cost of the complete microarraying apparatus. It is therefore desirable to provide a pin holder which provides a high degree of accuracy for the pin guidance and which can be fabricated more cheaply and easily than a conventional pin holder.
According to a first aspect of the invention there is provided a pin holder for a microarraying apparatus comprising: at least one group of bodies of circular cross-section packed together to form a network of pathways in gaps between the bodies; and an array of pins slidably arranged in at least a subset of the pathways.
With the invention, the pins are held parallel to each other automatically as a result of the self organized packing of the circular cross-section bodies. This is a great improvement over the prior art approach described above in which guide holes for the pins are bored or ground individually, and thus inherently will not be parallel to each other and will also suffer from eccentricity errors in the case that the pins are guided in two or more vertically displaced guide holes.
With the invention, any misalignment of the circular cross-section bodies will be collective, so that all the pins will be misaligned in the same way and thus remain parallel to each other. Such a misalignment will therefore cause no net effect on the spotting process.
In one embodiment, the at least one group of bodies comprises a group of spherical bodies arranged in a common plane, such as ball bearings. More specifically, first and second groups of spherical bodies are preferably arranged in first and second planes vertically displaced from one another.
In another embodiment, the at least one group of bodies comprises a group of cylindrical bodies, such as needle roller bearings.
With the invention, the principal contributory factor to irregularity in the pin alignment, and thus spot spacing, will be irregularity in the size of the circular cross-section bodies. However, ball bearings (i.e. spheres) or needle roller bearings (i.e. cylinders) are manufactured to a very high degree of dimensional uniformity and are mass produced items of low cost. They are also available in a variety of materials, such as stainless steel, tungsten carbide and ceramic. A pin holder with a 4xc3x976 pin array can be manufactured using conventional machining and by purchasing 70 ball bearings. This compares with having to jig grind the 48 holes needed for a conventional 4xc3x976 pin holder, as described above. The invention thus not only provides a technically superior solution in terms of pin alignment accuracy, but does so in a way which reduces the cost of manufacture of the pin holder by two orders of magnitude. Indeed the cost reduction of the pin holder that is realizable with the invention can impact significantly on reducing the total cost of an entire microarrayer.
With the invention, the pin outer surfaces are located by point contacts (as considered in plan view) between the outer surfaces of nearest neighbor circular cross-section bodies, with the number of point contacts being defined by the number of nearest neighbor circular cross-section bodies. With a square packing, the number will be four and with a hexagonal close packing, the number will be three. The point contacting of the pins within their guides greatly reduces friction of the pins, in comparison with the prior art approach of using circular cross-section guide holes in which a full circumferential portion of the pin""s outer surface is in contact with the whole inner surface of the guide holes.
If the circular cross-section bodies are spheres, then the contacting between the pins and guides will truly be point contacting. If two layers of spherical bodies are used, there will thus be eight points of contact in total for each pin. The use of spheres is considered to be the best mode of the invention, since any dirt or foreign bodies that find their way to the pin holder pathways will be self-cleaned away from the contact surfaces, and therefore not compromise pin alignment and motion. Alternatively, if the circular cross-section bodies are cylindrical, then the contacting between pins and guides will be line contacts. These will also be self-cleaning, but not to the same extent as with spherical bodies. Moreover, friction of the pins in the pathways will be larger than for spherical bodies.
The bodies are preferably packed together in a square grid. To provide a 4xc3x976 pin array, which is a standard, the square grid of bodies is preferably configured to provide a network of 4xc3x976 pathways, also conformant to a square grid. This can be done most efficiently when the square grid of bodies consists of a 5xc3x977 arrangement of bodies, also conformant to a square grid.
The standard spacing for microarraying is 4.5 mm. To design the pin holder to conform to this standard, the circular cross-section of the bodies preferably has a diameter of 4.5xc2x10.02 mm, 4.5xc2x10.01 mm, 4.5xc2x10.005 mm or 4.5xc2x10.0025 mm. A variance in the diameters of the bodies is preferably less than xc2x10.0025 mm, i.e xc2x12.5 xcexcm. With bodies of 4.5 mm diameter, the circular cross-section of the pins should nominally have a diameter of 1.864 mm from purely trigonometric considerations. However, to provide sufficient clearance for a smooth fit, the actual diameter should be somewhat less, namely 1.860xc2x10.02 mm, 1.860xc2x10.01 mm or 1.860xc2x10.004 mm. A desired diameter variance between pins of a certain specified diameter is xc2x10.004 mm, i.e xc2x14 xcexcm.
Accordingly, in a second aspect of the invention there is provided a set of pins for a pin holder of a microarraying apparatus, wherein each pin comprises a shank of circular section leading to a tip for carrying and dispensing liquid, wherein the shank has a diameter of 1.860xc2x10.02 mm, 1.860xc2x10.01 mm or 1.860xc2x10.004 mm, for example.
Other pin arrangements may also be used. For example, the pins may be arranged in a triangular or rectangular array, instead of a square array. This is possible by packing the bodies into a hexagonal close packed array, instead of square grid. Moreover, with a square grid of bodies, rectangular pin arrays can be provided by utilizing only a subgroup of the pathways.
According to a third aspect of the invention there is provided a spotting method comprising:
(a) providing a spotting head with a pin holder comprising at least one group of bodies having circular cross-section packed together to form a regular network of pathways in gaps between the bodies;
(b) providing an array of pins slidably arranged in at least a subset of the pathways; and
(c) selectively driving the pins in the pathways to deposit liquid on a spotting surface.
Further aspects of the invention relate to a microarraying apparatus and a head for a microarraying apparatus comprising a pin holder according to the first aspect of the invention.